As others have said, our reference of time comes from our own universe's rules.
Ergo if rendering 1 second of our time took 10 years of their time, we wouldn't measure 10 years, we'd measure 1 second, so we'd have no way of knowing.

It's worth remembering that simulation theory is, at least for now, unfalsifiable.
By it's nature there's always a counterargument to any evidence againat it, therefore it always remains a non-zero possibility, just like how most religions operate.